Co9Fe3Bi1Mo12O51 catalysts were prepared by a co-precipitation method at different pH (pH = 1-7), and were applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene in a continuous flow fixed-bed reactor. Conversion of n-butene and yield for 1,3-butadiene over Co9Fe3Bi1Mo12O51 catalysts showed volcano-shaped curves with respect to co-precipitation pH value. O-2-TPD (temperature-programmed desorption) measurements revealed that the catalytic performance of Co9Fe3Bi1Mo12O51 was closely related to the oxygen mobility of the catalyst. Yield for 1,3-butadiene was increased with increasing oxygen mobility of the catalyst. Among the catalysts tested, Co9Fe3Bi1Mo12O51 catalyst prepared at pH 3 showed the best catalytic performance due to its highest oxygen mobility. The pH value during the co-precipitation step strongly affected the oxygen mobility of Co9Fe3Bi1Mo12O51 catalysts, and in turn, the oxygen mobility played a key role in determining the catalytic performance of Co9Fe3Bi1Mo12O51 catalysts in the oxidative dehydrogenation of n-butene.